1. Field of the Invention
The present invention relates to a spherical aberration corrector for use in an electron microscope.
2. Description of Related Art
In a known aberration corrector for correcting spherical aberration in lenses included in an electron microscope, two axially symmetric lenses are placed between two multipole elements that produce hexapole fields. Note that the xe2x80x9caxially symmetric lensxe2x80x9d is so designed that the geometrical arrangement of lens properties is not affected by rotation of the lens about the optical axis. FIG. 3 schematically shows the configuration of the illumination system of an electron microscope fitted with the conventional spherical aberration corrector. Note that the deflection system and a part of the focusing system are omitted from this figure. The microscope has a source 1 emitting an electron beam 2. This beam 2 passes through a condenser lens 4 having an aperture 3. The beam made parallel to the optical axis enters spherical aberration correction optics 5. An electron beam exiting from the correction optics 5 and traveling parallel to the optical axis is directed onto a specimen 7 through an objective lens 6.
The spherical aberration correction optics 5 comprise multipole elements 8 and 9 for producing the hexapole fields, and axially symmetric lenses 10 and 11 located between the multipole elements 8 and 9. These multipole elements 8 and 9 are so arranged that they are in phase with respect to the optical axis and have no rotational relation about the optical axis within a plane perpendicular to the optical axis. The lenses 10 and 11 have the same focal length of f. Their spherical aberration is corrected provided that the distance between the multipole element 8 and the axially symmetric lens 10 is f, the distance between the lenses 10 and 11 is 2 f, the distance between the lens 11 and the multipole lens 9 is f, the multipole elements 8 and 9 are excited with the same intensity K, and the elements 8 and 9 have the same width Z as measured along the optical axis.
With the conventional spherical aberration correction optics, however, it is necessary to realize certain arrangement conditions using axially symmetric lenses of the same focal length. Therefore, it is impossible to vary the magnification by means of the correction optics. Accordingly, the obtained minimum electron probe is limited by spherical aberration. Consequently, it is impossible to obtain a sufficiently small electron probe having a sufficient amount of current. The demagnifying action on the electron probe needs to be assigned to other lenses.
Furthermore, the multipole elements 8 and 9 need to be so arranged that there is no rotational relation about the optical axis within a plane perpendicular to the optical axis. In practice, a certain degree of rotational relation is inevitably introduced within manufacturing and assembly tolerances. In addition, electrons transmitted through the axially symmetric lenses 10 and 11 undergo a rotating action within the plane perpendicular to the optical axis. If the polarity of any coil is reversed, a certain degree of rotational relation is unavoidably introduced. Therefore, it is necessary to correct the introduced rotational relation by controlling the excitation of the multipole elements and rotating the phase angle of the acting field. Where the multipole elements are used in this way, they will easily produce higher-order aberrations.
It is an object of the present invention to provide a spherical aberration corrector which is for use in an electron microscope and which permits the magnification to be varied by means of spherical aberration correction optics. Furthermore, the corrector can correct the rotational relation between multipole elements within a plane perpendicular to the optical axis without varying the phase angles of the multipole elements. For this purpose, the spherical aberration corrector built in accordance with the present invention has a rotation-correcting lens between two axially symmetric lenses located between the two multipole elements. This corrector is characterized in that the rotation-correcting lens is positioned within the focal plane of an electron trajectory formed between the axially symmetric lenses to rotate electrons within the plane perpendicular to the optical axis.
The invention also provides a spherical aberration corrector which is for use in an electron microscope and has two axially symmetric lenses (i.e., a front-stage lens having a focal length of f1 and a rear-stage lens having a focal length of f2 different from f1, i.e., f1xe2x89xa0f2) between two multipole elements (i.e., a front-stage multipole element and a rear-stage multipole element), the corrector being designed such that the distance between the front-stage multipole element and the front-stage lens is set to f1, the distance between the two axially symmetric lenses is set to f1+f2, the distance between the rear-stage lens and the rear-stage multipole element is set to f2, the excitation intensity of the front-stage multipole element is set to K1, and the excitation intensity of the rear-stage multipole element is set to K2. The lengths of the front-stage and rear-stage multipole elements as measured along the optical axis are Z1 and Z2, respectively. The corrector is further characterized in that it is designed to satisfy the relations             Z      2        =                  a        2            ⁢              Z        1                        K      2        =                  K        1                    a        5                        w      ⁢              xe2x80x83            ⁢      h      ⁢              xe2x80x83            ⁢      e      ⁢              xe2x80x83            ⁢      r      ⁢              xe2x80x83            ⁢      e      ⁢              xe2x80x83            ⁢      a        =                  f        2                    f        1            
Other objects and features of the present invention will appear in the course of the description thereof, which follows.